The present disclosure relates generally to well logging and measurement in subterranean formations and more particularly, the present disclosure relates to a system and method for identifying anisotropic formations, such as fractures and fracture patterns, in subterranean formations.
Acoustic well-logging may be used to determine fracture patterns or anomalies in subterranean formations. Orthogonal acoustic wave propagation and its polarization are used to determine fracture direction—also known as “seismic anisotropy.” Industry professionals may use this information for hydrocarbon or mineral extraction, for instance and while fractures can indicate hydrocarbon or mineral deposits, not all anisotropic formations function as indicators of deposits. Furthermore, fracture direction may provide essential information on the direction and extent of drilling for extraction of materials from subterranean formations.
Historically, seismic anisotropy technology used refraction of compressional wave propagation to determine the azimuth of the fracture. The compressional wave would refract in a chevron pattern when hitting an anisotropic subterranean formation. The refraction, however, occurs when encountering any anisotropic formation, not just fracture-related anisotropic formations. Furthermore, the historical method relies on wavelength for finding the fracture. The wavelength of compressional waves is considered “long”—it is often referred to as a “slow wave”—thereby limiting the size of the fracture or anisotropic formation detectable to larger fractures. Due to the ambiguity and the lack of precision available from use of the historical method, it was abandoned for newer technology.
Currently, seismic anisotropy technology uses a method that interprets shear wave propagation to determine the azimuth of the fracture. In anisotropic formations, shear waves split into fast and slow components that move in orthogonal directions. The difference between the fast and slow components indicates the degree of anisotropy. Fractures are anisotropic, so shear wave splitting can be an indication of fractures in subterranean formations; however, not all anisotropic formations are fractures. Therefore, use of shear wave anisotropy is not so much a definitive indicator of fractures, but a definitive indicator of anisotropic formations that may be fractures but may be alternative anisotropic, or seemingly anisotropic, formations. This demonstrates some key limitations to the use of shear wave anisotropy that are becoming more pronounced as hydrocarbon extraction involves more anomalous and tighter subterranean formations. Furthermore, fractures, though anisotropic, can also exist in mildly anisotropic formations that cannot be detected by the present-day methods, as it is difficult to distinguish the subtle fracture patterns from the surrounding anisotropic media using shear waves.
In situations where energy mapping from shear wave anisotropy fails to distinguish between surrounding rock formations or where a redundant method to verify the shear wave results is asked for, shear sonic imaging is sometimes used to identify anisotropic formations in surrounding subterranean formations. The imaging technology relies on a difference of resistivity between the fluid in the well bore and the surrounding rock formations. The larger the difference in resistivity, the clearer the image produced should be. With softer rock formations, such as shale, the resistivity between the rock and the fluid in the well bore is not significantly different. Therefore, the current imaging software may not be able to provide clear pictures of anisotropic formations when used on soft rock formations.
Due to the drawbacks of the current and past methods, there still exists a need in the market for a method or technology to address these concerns and to improve the overall efficiency of fracture identification in subterranean formations.